High-performance display devices, such as liquid crystal (LC), organic light emitting diode (OLED), and plasma displays, are commonly used in various electronics, such as cell phones, laptops, electronic tablets, televisions, and computer monitors. OLED light sources have increased in popularity for use in display and lighting devices due to their improved color gamut, high contrast ratio, wide viewing angle, fast response time, low operating voltage, and/or improved energy efficiency. Demand for OLED light sources for use in curved displays has also increased due to their relative flexibility.
A basic OLED structure can comprise an organic light emitting material disposed between an anode and a cathode. The multi-layer structure can include, for example, an anode, a hole injection layer, a hole transporting layer, an emitting layer, an electron transporting layer, an electron injection layer, and a cathode. During operation, the injected electrons from the cathode and holes from the anode can be recombined in the emitting layer to generate excitons. When current is supplied to the organic light emitting material, light is given off due to the radioactive decay of the excitons. To form a display device comprising an OLED, a plurality of anodes and cathodes can be driven by a thin film transistor (TFT) circuit. The TFT array thus provides an array of pixels which can then be used to display selected images by the application of current through the anodes and cathodes.
While OLED display devices may have numerous advantages over other display devices, such as LCDs, OLEDs may still suffer from one or more drawbacks. For example, OLEDs can have limited light output efficiency (luminance) as compared to other light sources. In some instances, as much as 80% of the light energy emitted by the OLED may be trapped in the display device. Light generated by the emitting layer can, for instance, be confined within the electrode and glass substrate of the device due to a large difference in refractive index (n) values for these layers (e.g., ne≈1.9, ng≈1.5). Snell's law suggests that the difference in refractive indices produces a low out-coupling efficiency in the range of about 20%, where the efficiency level is expressed as the ratio of surface emission to the total emitted light. Thus, even though internal efficiencies nearing 100% have been reported, the low out-coupling efficiency ultimately limits the brightness and efficiency of the OLED device.
Numerous methods for improving light extraction efficiency of OLED devices have been proposed, including high index substrates and particles and/or various surface modifications. However, these techniques may require expensive materials and/or complex processes, such as photolithography and the like, which can unnecessarily increase the manufacturing time and overall cost of the device. Attempts to increase the light output of an OLED device have also included driving the OLED at relatively high current levels. However, such high currents can have a negative impact on the lifespan of the OLED and thus also fail to provide an ideal solution.
Other attempts to improve light extraction efficiency include, for example, waveguides that are matched to the OLED layer in thickness and/or index, such that modes within the OLED can be matched with modes within the waveguide. Such waveguides can be deposited on a glass substrate and subsequently coated with a planarizer (e.g., smoothing) layer. Improved light extraction has been observed with relatively thin planarizer layers (e.g., less than about 0.5 microns). Thicker planarizer layers may, for example, yield an insufficiently small overlap between the evanescent OLED light and the waveguide modes. However, thinner planarizer layers may result in an overly rough interface between the waveguide and OLED layer, which can cause coupling within the modes of the OLED, such that light can couple from one of these propagating modes to a surface plasmon mode (or surface plasmon polariton). Surface plasmon modes are highly absorbing and, thus, coupling of light to these modes is typically undesirable. Moreover, waveguides including planarizer layers require multi-step processing, which can increase manufacturing time and/or cost.
Accordingly, it would be advantageous to provide waveguides for display (e.g., OLED) devices that can provide improved light extraction efficiency while also reducing the cost, complexity, and/or time for manufacturing the device. Additionally, it would be advantageous to provide waveguides having a desirable surface roughness without the use of a planarizer layer. Furthermore, it would be advantageous to provide methods for making waveguides using a single-step coating process.